1. Field of the Invention
This invention relates to hydraulic workover equipment for use in servicing oil and gas wells, and more particularly, to a blowout safety system suitable for use during snubbing operations.
1. Description of Related Art
As tubular members such as pipe or reeled tubing are run into a well during snubbing operations or other well servicing procedures, pockets of gas may be encountered that produce surges in well pressure. Whenever the downhole pressure exceeds the weight of the pipe in a well bore, sometimes referred to as the "light pipe" mode, positive force must be exerted on the pipe to prevent it from being blown out the top of the well. This positive force is normally exerted on the pipe by means of a hydraulic jack comprising sets of diametrically opposed hydraulic cylinder assemblies having piston rods that raise and lower upper traveling slips. Two sets of lower slips are mounted in axial alignment below a guide tube assembly, one for holding the pipe against the well pressure and one for supporting the weight of the heavy pipe while resetting the upper traveling slips. The hydraulic cylinder assemblies of the jack are operated from a control panel that is situated on the work platform or basket of the workover rig, or located nearby.
If the grip on the pipe is lost due to a sudden pressure surge while operating in the light pipe mode, the well pressure can force the pipe upward and out of the well bore through the center of the hydraulic jack. Although conventional blowout preventers are utilized during snubbing operations, well fluids and acids may escape upward through the center of the jack, thereby endangering personnel on the work platform before the blowout preventers can shut down the well. Blowout preventers are normally hydraulically operated, and may require several seconds to seal off the well and stop the flow. For the protection of personnel and equipment, a blowout safety system is therefore needed that is characterized by a safety valve having a shorter response time than that of the conventional, commercially available blowout preventers.
U.S. Pat. No. 4,085,796 discloses a well tubing handling system comprising a hydraulic jack and slip assembly of the type suitable for use in snubbing operations.
U.S. Pat. No. 4,473,122 discloses a downhole safety system comprising a valve that is set downhole to prevent the undesired escape of well fluids while moving a work string through the bore of a production tubing string. The downhole safety valve has a flapper closure means that can be mechanically opened by inserting a work string through the safety valve and that will close when the work string is withdrawn. The flapper is biased toward the closed position by a torsion spring disposed on the hinge pin, and when closed, seals off fluid flow through the production tubing.
U.S. Pat. No. 4,889,184 discloses a wellhead assembly for use with a reciprocating rod pump. The assembly comprises a polished rod stuffing box with a safety valve for sealing off the wellhead in case of polished rod failure. The safety valve utilizes a flapper that is biased against the polished rod by a torsion spring. When the polished rod is removed, either intentionally or through failure, the flapper seals off the wellhead. In such wells, however, there is typically little if any downhole pressure to be retained, and the primary function of the safety valve is to prevent the escape of potentially dangerous gases.
U.S. Pat. No. 4,907,650 discloses another wellhead assembly for use with a reciprocating rod pump. The assembly comprises a flapper valve that is normally spring biased toward the open position, and that is closed by a plunger attached to a transversely mounted piston adapted to force the flapper closed against the spring bias in the event of polished rod failure or removal.